This invention relates to a compact fluorescent lamp with a housing structure, and, more particularly, to a structure for drawing excess heat from the discharge tube of the compact fluorescent lamp.
Mercury dosed single-ended low-pressure discharge lamps, commonly referred to as compact fluorescent lamps, have a widespread use both for indoor and outdoor lighting. The advantage of these light sources is that they can be operated more economically than other conventional light sources due to their long life and their low power consumption. The luminous output of these lamps however highly depends on the partial pressure of the mercury which is effected by the temperature of the discharge space. Due to a higher ambient temperature, the discharge space may be overheated which causes the partial pressure of the mercury to rise above an optimum value. Consequently, the luminous flux of the lamp will be lower than the luminous flux available at optimum temperature range.
Several constructions have been created for obtaining the required pressure of the mercury so far. The principle of these constructions is that the partial pressure of the mercury can be adjusted by producing a so-called cold spot in the discharge tube where the excess mercury condenses so that the luminous efficiency approaches the optimum value. U.S. Pat. No. 5,717,277 describes a construction for compact fluorescent lamps in which vertical and horizontal insulating elements are positioned in the housing of the lamp in order to provide a cold spot in a tubulation placed at the end of the discharge tube. U.S. Pat. No. 4,694,215 offers a different construction which defines the mercury vapor pressure by providing for a predetermined temperature cold spot within the fluorescent lamp. The lamp allows vertical cooling air to stream from a lower opening to an upper outlet in the housing. In the so-called xe2x80x9cbase-downxe2x80x9d operation, in which the lamp operates in vertical position so that he base is the lowest point, the mercury within the fill of the fluorescent lamp may condense. The mercury collects as mercury drops in the cooled tubulation thereby stabilizing the vapor pressure within the discharge vessel.
A disadvantage of these fluorescent lamps is that the location of the cold spot varies according to the operating position of the lamp. The operating position has a great impact on the luminous output of the lamp. Another disadvantage is that the mercury, which continuously vapors and condenses, may, under some operating conditions, even make flaking or removal of the fluorescent material with which the inside of the discharge vessel is coated. Furthermore, the optimum temperature range of such lamps is narrow for many applications. In order to overcome this difficulty, fluorescent lamps with amalgam droplets were developed in which the excess mercury is amalgamated in these droplets. Fluorescent lamps using amalgam present a wider optimum temperature range allowing a broader scale of application.
In many cases, this broader scale is not sufficient especially when the lamp is located in recessed fixture or it is furnished with outer bulb which increase the ambient temperature locally. U.S. Pat. No. 4,794,301 describes a fluorescent lamp with outer bulb which incorporates a quantity of amalgam held within a tubulation placed at the end of the discharge tube. The tubulation protrudes into an air chamber on the wall of which inlets and outlets are formed in order to maintain air ventilation for cooling the amalgam. A disadvantage of these lamps is, that communication with the atmosphere requires inlets and outlets which prevent the lamp from being used in outdoor application. Another drawback of the compact fluorescent lamps using ventilation slits that they can meet the safety requirements only by meticulous design. Depending on the application, i.e. the ambient temperature and the fixture type in which the lamp is used, a very wide optimum operation temperature range is needed. Comparing a stand-alone lamp operating at ambient temperature of 0xc2x0 C., e.g. in a garage, and a lamp in a recessed fixture, e.g. on a ceiling, the ambient temperature ranges from 0xc2x0 C. to 70xc2x0 C. Practically, compact fluorescent lamps with amalgam cannot even cover this wide temperature range.
Thus there is a particular need for a compact fluorescent lamp structure which enables the lamp to operate in a very wide range of optimum temperature. It is therefore desirable to provide a compact fluorescent lamp with amalgam the thermal optimum of which can be shifted towards higher ambient temperatures and which maintains the optimum luminous output in a larger variety of applications.
In an exemplary embodiment of the present invention, a compact fluorescent lamp comprises a plastic housing. The plastic housing includes a plastic cap and a plastic shell. A discharge tube extends from the plastic housing through openings formed in the plastic cap. The compact fluorescent lamp further comprises a tubulation which contains amalgam material and communicates with the discharge tube. A heat conducting means is integrally formed with the plastic housing which is suitable for receiving a heat conductive material forming a thermal bridge towards the tubulation.
This construction has the following advantages over the prior art. When the heat conducting means integrally formed with the plastic housing receives a heat conductive material, the plastic housing serves as a heat sink and heat radiating body which allows the lamp to operate with optimum luminous output at higher ambient temperatures. This offers several additional possibility of application for the lamp, e.g. in recessed ceiling or built-in fixtures in counter-ceilings. The heat conducting means may be filled with heat conductive material easily before the final step of lamp assembly. Since the presence of the heat conductive material is optional, a wide ambient temperature range with optimum luminous output is accomplished. The abandonment of the heat conductive material results in a lamp operating with optimum luminous output in a lower ambient temperature place, e.g. in a garage.